Well World

The setting for two separate series of science fiction novels by Jack Chalker, beginning with Midnight at the Well of Souls in 1977. The series and the volumes in them are:

The Well of Souls series:

Midnight at the Well of Souls (1977)
Exiles at the Well of Souls (1978)
Quest for the Well of Souls (1978)
The Return of Nathan Brazil (1980)
Twilight at the Well of Souls (1980)
The Sea is Full of Stars (1999)
Ghost of the Well of Souls (2000)

The Watchers at the Well trilogy:

Echoes of the Well of Souls (1993)
Shadow of the Well of Souls (1994)
Gods of the Well of Souls (1994)

The Well World is a planet-sized computer created by an ancient race that used it to expand and stabilize the universe; its surface is a series of 1560 hexagonal laboratories used to develop new sentient species with carbon-based lifeforms in the southern hemisphere and truly alien lifeforms in the north. Newly arrived space travelers are incorporated into the experiment by being automatically assigned to a "hex" and transformed into its dominant species. Control the Well World and you can theoretically control the entire universe. Of course, it's never that simple.

Tropes used in Well World include:

Action Survivor: Nathan Brazil. Despite being the immortal guardian of the universe, he's small, unimposing, and doesn't normally go looking for trouble. His survival is guaranteed but his integrity is not and he's been mutilated quite a bit over the years, and while the stuff he loses always grows back it's a long and painful process.
All Myths Are True: Centaurs, fairies, fauns are real alien races. Earth -- hell, the entire universe -- really was created in seven days.
And I Must Scream: The final fate of Juan Campos, the South American rapist drug lord from the Watchers at the Well trilogy. Nathan Brazil decides a fitting fate for Campos is to be dropped into the body of a gorgeous, mute South American Indian woman, who has wandered a bit too close to the Campos compound around the time that the Family's henchmen are looking for some fun. To add squick to the matter, Brazil's modus operandi is to find people who have recently died, and stick the soul of the person he's dealing with into their reanimated body. Campos, my boy, you are now a zombie sex slave for the family you were once groomed to lead. And only you will ever know or believe it. Sleep well.
Author Appeal - Even a casual survey of Chalker's work will show that he was obsessed with the idea of humans changing their species and sex. The Well World books just gave him a justification for doing it on a massive scale.
- Not to mention the recurring cases of strong-smart-woman-turned-virtually-helpless-by-vision-or-reading-impairment-and-body-morphing. Seems to happen in the majority of Chalker's books.
Bizarre Alien Biology - almost sixteen hundred hexes' worth.
Brain In a Jar: Nikki Zinder, eventually.
Charles Atlas Superpower - Gypsy (Dr. Zinder) has learned to directly manipulate minor local aspects of the universal programming, without technological assistance -- he claims it's all done mentally using mathematics.
Complete Immortality - Nathan Brazil, among other names he uses; later, Mavra Chang.
Cosmic Retcon: Nathan Brazil has had to reboot the entire universe at least half a dozen times -- and notes that although the universal program is rigidly-defined enough that (for instance) a recognizable Earth will always develop, things still turn out slightly differently each time, as the starting conditions vary.
Did Not Do the Research: One of Nathan Brazil's memories is of taking part in the Maccabean Revolt -- against The Roman Empire. The Maccabees actually revolted against the Seleucid Empire ... and sent an embassy hoping for an alliance with The Roman Republic. Or perhaps that's something that's been reset....
Doomsday Device: Zinder Nullifiers, created by blindly hacking Zinder's discoveries centuries later as a last-ditch weapon against an invading alien force. Unfortunately, using them tears a hole in the fabric of the universe so bad that the Well World can't repair it -- so it sends a recall signal to Nathan Brazil.
Eldritch Abomination: Basically what a Markovian looks like. Imagine a 15-foot-tall human heart walking on six very muscular tentacles that sprout from its equator.
Enforced Technology Levels - Each hex on the Well World has a maximum technology level -- Non, Semi, or High -- to simulate the availability of resources in that race's target biosphere. Technology above a hex's maximum level will simply fail to work inside that hex.
The Fair Folk: An insectoid race that can abuse physical laws by virtue of their innate ability to make minor changes in reality's programming, essentially making them magical.
Fate Worse Than Death: Nikki Zinder is one of the founders of the Olympians, the race that Ben Yulin created through Obie's powers. When the other founders figure out they're all reaching their age limits, they go back to the Well World for one last life time. Nikki, on the other hand, uses cybernetics to stay alive and becomes a brain in the jar to continue leading her people. When you first see her in The Return of Nathan Brazil, she seems relatively sane until Mavra leaves the room. Then she goes on a religious rant and orders her people to make sure Brazil doesn't fall into the "Evil One's" hands, before laughing madly and then begging in a child's voice for her father to come back to her, despite the fact that she can't reasonably think he's still alive.
- He is. He has to Mercy Kill her or her insanity will get in the way of saving the universe.
Fisher Kingdom - entering the Well World eventually requires transformation into one of that world's indigenous races.
The Fog of Ages - In the first book, Nathan Brazil is so old that he's forgotten most of his life, including his own origins; it all comes flooding back when he comes (back) to the Well World.
Gender Bender - numerous
The Great Politics Mess-Up - Later revised as the universe is rebooted. In earlier Well World stories, communism spread to the stars and there were even people who worshiped Marx and Lenin. After the universe was rebooted, the stories which (partially) took place on modern Earth noted that things are different.
Hermaphrodite - several species, mostly aquatic.
- As well as variant humans, usually implemented by one or another of the Com worlds in the interest of "equality".
Humans Are White: Averted with Nathan Brazil, who is described as having a dark complexion, black hair, brown eyes, and a Roman (aquiline) nose, mostly evoking an Indian-ish appearance.
- Also, it's noted that the humanity of Midnight's far future is on the whole vaguely Asian-looking, although the degree to which this is true varies from world to world.
Hurricane of Puns - Those hex names which aren't Shout-Outs or nonsense words. Like "Wasdamaddah". (Surely the site of a university of some reknown.)
Inhumanity Ensues
Laser-Guided Karma - If you make it to the Well of Souls with Nathan Brazil, better hope you were a good person.
Magic Versus Science - In some hexes the differing laws of physics allow "magic" of various sorts, which is in fact all just the Master Computer doing its thing.
Master Computer - for the entire universe.
The Mind Is a Plaything of the Body: Actually averted, as multiple mention is made of how a person's mind has to be altered to make them comfortable in their new forms.
Mister Seahorse - literally (via Mix-and-Match Critters)
Mix-and-Match Critters- justified as lazy Markovians cribbing each other's work all those eons ago.
Multiple Choice Past: Brazil has at various times claimed to be God, Moses, the last member of a race of Precursors, or an ordinary human who was tricked into become the caretaker of the universe. It's actually the third.
No Sex Allowed- Taken to extremes by the Comworlders, most of whom are genetically or biochemically engineered to never hit puberty.
One-Gender Race - the Czillians, a species of sentient, parthenogenic plants
Our Dragons Are Different
Power Perversion Potential - Obie's an AI supercomputer hooked up to a Matter/Energy/Matter converter and planted inside a planetoid big enough to house a small town. He can give you anything you want, as long as he knows how to make it, and has a big transmitter capable of altering entire planets to a person's specifications. He's also capable of traveling to anywhere he knows the coordinates of within minutes. Upon hearing of Obie's full power, Nathan Brazil himself said, "to hell with porn," and upon seeing Obie in action was rather disturbed.
Precursors: The Markovians, who were the first (and only) race in the Universe. They felt that even though they had reached godlike heights, they had missed something crucial in their development. The Well World and the races developed there were their attempts to try something different that just might find what they had missed.
Ragnarok Proofing - The Well World itself. Not only is it a planet-sized computer that has been functioning for trillions of years since its creators disappeared, but if true Ragnarok is at hand you can just ctrl-alt-del the thing and reboot the entire universe.
Rewriting Reality: The raison d'etre for the Well World. The same mathematics that it uses can also grant this power to individuals who understand it.
Sense Freak/Showing Off the New Body as each character awakens to discover their new species.
Second Law of Gender Bending: also applies to all of the species bending, as well.
Shout Out: Plenty, as shown in entries such as Tuckerization below. Also, one villain is named after the extraterrestrial equivalent of Santa Claus from the novel Wasp by Eric Frank Russell; Chalker strongly admired Russell's work.
Starfish Aliens: Well World is divided into two parts. Southern Hemisphere is for carbon-based lifeforms, and a lot of those are quite starfish-looking. Northern Hemisphere is for non-carbon-based ones, and those are just plain weird -- even to each other.
Steampunk - in certain hexes, technology is limited to steam.
Sufficiently Advanced Aliens - the Markovians.
Tuckerization - Many hex names are Shout-Outs to people Chalker knew in SF fandom and the publishing industry.
Unreliable Expositor: Nathan Brazil, at least in regards to his own personal history.
What If God Was One of Us? - Maybe, but also practically yes. Nathan Brazil's real origin is never stated, and several of the characters wonder about him: is he really God, the last Markovian, or a construct of the Well itself? At the end of the day, it doesn't matter because once he's in the Well Computer, he is a deity.
- The last book reveals he is indeed the last Markovian.
What Measure Is a Non-Human?
Who Wants to Live Forever? - Brazil, in his darker moments. But what do you expect from someone who's been forced to destroy and recreate the universe at least five times?
Writers Cannot Do Math - Look at the partial maps of the Well World and the data given in the text. Try to extrapolate the possible numbers of hexes. 1560 is not a possible result.